The invention concerns a method as well as an apparatus for the determination of measurement uncertainties in a device for X-ray fluorescence layer thickness measurements for the measurement of a layer of a sample under investigation.
Devices for measuring the thickness and also the composition of layers, in particular, galvanic layers in the micrometer range but also of alloy layers using X-ray fluorescence often have X-ray tubes, a detector as well as an observation device having a focusing element. When using such a device it is necessary to determine the suitability of the apparatus for the requirements at hand. Quality control has therefore introduced the terminology "measuring means capability". Towards this end a limiting value is defined, the observance of which should guarantee the suitability of the apparatus for the application. The central question underlying this estimation of the suitability of the apparatus to be used is the precision with which a particular measuring quantity, in this case the layer thickness, can be determined using the device for X-ray fluorescence layer thickness measurements. In addition one has to determine whether or not this precision is sufficient to carry out the measurement within certain tolerances. In devices for measurement of layer thicknesses and apparatus therefor, this requirement consists of determining the measureability of certain layers or layer combinations of the sample under investigation. Towards this end at least a certain given precision must be achieved in certain thickness regions.
If the suitability of such a device has to be determined empirically each time, this would require substantial effort in particular with regard to a suitable sample material, processing time etc. In addition, for the case of new types of layer technology, such a suitable sample material is often not available at all.
For this reason certain points of departure have been used to calculate and determine the measurement uncertainty and the measuring time in such devices. Towards this end the sum of all events registered by the detector within a certain pulse height interval, described as a signal magnitude X, the measurement time t, as well as a function f describing the interdependence between the measured quantity d and the signal X are used to estimate the theoretical scatter .sigma.(d) in accordance with EQU .sigma.(d)=f'(X)*.sigma..sub.X
Such a starting point is only possible if the pulse height interval considered does not overlap with interfering spectral regions during the measurement. This is however the case in complex measuring problems as well as during the measurement of a plurality of layers or alloy layers. The mutual influence of the individual layers during the fluorescence excitation leads to non-linear dependencies in the relationships between the layer thickness and counting rates X.
It is therefore the underlying purpose of the invention to create a method as well as a device of the above mentioned kind by means of which the measurement uncertainty can be determined without a comprehensive empirical experimental phase and in a reliable manner even for complicated measuring situations, such as multiple layers or alloy layers.